Seismic shuttle logging tools are know for use in performing vertical seismic profile surveys from boreholes to evaluate the surrounding underground formations. A multi-shuttle seismic tool comprises a number of similar tools (shuttles) linked together by means of a cable and logged through a borehole while seismic signals are generated at the surface. At each measurement level, each shuttle is anchored to the wall of a borehole during a measurement operation by means of an anchoring arm. In VSP surveys, it is necessary to make measurements at a number of levels in a borehole so the tool anchoring must be released to allow the tool to be moved and then re-engaged at the next level. The time taken for the anchor to release and to re-engage can be a significant factor in the time taken to make a measurement at each level. In Schlumberger's Combinable Seismic Imager tool (CSI) the anchoring arm of each shuttle extended under spring bias and a drive motor is used to provide the anchoring force or retraction. Aspects of the CSI are described in U.S. Pat. No. 4,563,757; U.S. Pat. No. 4,575,831; U.S. Pat. No. 4,987,969; and U.S. Pat. No. 5,200,581. In particular, patents '757 and '831 relate in particular to the anchoring mechanism and procedure. A schematic figure from the patents is shown in FIG. 1. This prior art arrangement includes a motor 35, an output reducer 36 with an electromagnetic brake 37 and an output shaft to a coupling device 39 comprising a clutch 50, a mechanical logic 51 and torque limiter 54. The mechanical logic 51 includes studs located in helical, V-shaped, cam slots which serve to bring the clutch into engagement on operation of the motor. Drive is transmitted to a push rod 34 connected to the anchoring arms 31, 32 by means of a ball screw 40 and nut 41. The anchoring arms are urged away from the tool body 29 by a leaf spring 43 which is fixed to the tool body 29 and bears upon the arm 31. When in the open position, a pad 30 at the end of the arms 31, 32 engages the borehole wall and causes the tool body 29 to be pushed against the opposite side of the borehole where it can be anchored for use. The motor 35 is used to provide the extra anchoring force to the arms 31,32 and to withdraw the arms when the tool is run in or pulled out of the borehole. When the tool is to be moved to a different level, the action of the clutch 50 allows the motor to be disengaged and the arms held only by the force of the leaf spring 43.
The present invention has as its object a tool which has an anchoring arm which can be anchored and released in a relatively short time and in which the anchoring mechanism can be relatively compact.